What it is
Oleth-10 is derived from Oleyl Alcohol. Although it naturally occurs in fish oils, the type used in cosmetics is usually synthetically derived.
What it does
Oleth-10 is a surfactant: it is used in products to cleanse hair and skin because it allows water to mix with oil and dirt, rinsing them away and leaving you squeaky clean.
In addition, it also helps to form emulsions by allowing ingredients that normal wouldn’t mix together (like water and oil) to hold together and not separate.
Oleth-10 is mildly comedogenic and irritating.